Hi Sven,

Personally, I do not like the idea of having to generate the config files
on every host - I prefer to tangle once, on my main machine, and then
distribute (via git) both the org source and the tangled result to all my
machines. This results in much lower maintenance effort - otherwise I would
need to login to each machine, open Emacs there, and tangle the file (or
automate the process, but still, more work than doing it once). This means
that the logic still needs to be present in the resulting file, but it
doesn't mean the org source needs to be less readable. What I do is use
noweb mode selectively, to indicate the corresponding sections. I use this
for the OS-specific configuration in my Emacs file:
https://github.com/zzamboni/dot-emacs/blob/master/init.org#system-specific-configuration
.

In short, the top-level block includes the necessary logic, plus the
corresponding sections in noweb-style references (the :noweb no-export
makes it so that the tags get expanded only when tangling and not when
exporting, which makes my rendered config files more readable, see the
links below):


#+begin_src emacs-lisp :noweb no-export
  (cond ((eq system-type 'darwin)
         <<Mac settings>>
         )
        ((eq system-type 'windows-nt)
         <<Windows settings>>
         )
        ((eq system-type 'gnu/linux)
         <<Linux settings>>
         ))
#+end_src


Then, each subsection sets the corresponding section-level properties, so
that they don't have to be specified in each block - then all the blocks
within that section will be automatically inserted in the corresponding
"section" of the cond statement above. For example:

** Mac
:PROPERTIES:
:header-args:emacs-lisp: :tangle no :noweb-ref Mac settings
:END:

First, we set the key modifiers correctly to my preferences: Make Command
(⌘) act as Meta, Option as Alt, right-Option as Super

#+begin_src emacs-lisp
  (customize-set-variable 'mac-command-modifier 'meta)
  (customize-set-variable 'mac-option-modifier 'alt)
  (customize-set-variable 'mac-right-option-modifier 'super)
#+end_src


You can see the real file here:
https://github.com/zzamboni/dot-emacs/blob/master/init.org#system-specific-configuration,
or in a nicer rendering in my blog:
https://zzamboni.org/post/my-emacs-configuration-with-commentary/#system-specific-configuration

Hope this helps - not exactly what you asked, but maybe a useful
alternative?

Best,
--Diego


On Sat, Jul 28, 2018 at 11:25 AM, Sven Bretfeld <sven.bretfeld@ntnu.no>
wrote:

> Hi
>
> I don't know how you guys maintain init files for different hosts. I
> have one org-file with the header:
>
> #+PROPERTY: header-args :tangle ~/.emacs
>
> The file is synced to all my machines and produces the local init files
> on each. Most configurations are shared, but some are host-specific
> (e.g. font size).
>
> Sadly export filtering does not work with the tangle function. It would
> be nice to be able to do something like:
>
> ,----
> | * Default Frame
> | ** Office Computer                            :OFFICE:
> | #+begin_src emacs-lisp
> | (setq default-frame-alist '(
> |         (font . "-PfEd-DejaVu Sans Mono-normal-normal-normal-*-
> 26-*-*-*-m-0-iso10646-1")
> |         (width . 102)
> |         (height . 41))
> | #+end_src
> |
> | ** Computer at home                           :HOME:
> | #+begin_src emacs-lisp
> | (setq default-frame-alist '(
> |         (font . "-PfEd-DejaVu Sans Mono-normal-normal-normal-*-
> 18-*-*-*-m-0-iso10646-1")
> |         (width . 150)
> |         (height . 50))
> | #+end_src
> |
> | ** Laptop                                     :LAPTOP:
> | #+begin_src emacs-lisp
> | (setq default-frame-alist '(
> |         (font . "-PfEd-DejaVu Sans Mono-normal-normal-normal-*-
> 12-*-*-*-m-0-iso10646-1")
> |         (width . 80)
> |         (height . 30))
> | #+end_src
> `----
>
> It should be clear what this is about. On the office computer you would
> prepare the file headers to exclude the tags HOME and LAPTOP from being
> tangled and, after saving/tangling the file, you would have a nice init
> file suiting this computer. At home the same by excluding the other tags
> etc.
>
> This would save a lot of work and you would have a tidy way to maintain
> all your init files without (if (string-equal (system-name) clauses.
>
> If the noexport tag worked, it would also save a lot of time and mess
> when debugging your init file in case of an error.
>
> I don't actually understand why the developers decided not to implement
> export filtering in the tangling operations. I know about the COMMENT
> keyword, but the above example should make clear that this solution is
> far from handy.
>
> Maybe there is another way that escaped me so far?
>
> All best,
>
> Sven
>
>
>